1. Technical Field
The invention relates to a method for driving a CCD (Charge Coupled Device) type solid-state imaging device and an imaging apparatus, and more particularly, to a method for driving a CCD-type solid-state imaging device and an imaging apparatus, which are suitable to reduce a dark current component contained in a low luminance image portion of a taken image.
2. Description of the Related Art
CCD-type solid-state imaging devices are configured to transfer signal charges detected by photoelectric conversion elements via charge transfer paths, for output. Therefore, a dark current may be mixed into the signal charges during the charge transfer. The influence of the dark current increases when a signal component, i.e., a signal charge amount is small, while the dark current has less influence on a high luminance image formed from a large amount of signal charges.
JP 2005-286470 A describes that when it is determined based on an imaging condition such as ISO sensitivity that an incident light amount is small a camera photographs a subject, the capacity of potential packets for charge transfer, i.e., the number of transfer electrodes is decreased. On the other hand, when it is determined that the incident light amount is great, the capacity of potential packets is increased. In this manner, JP 2005-286470 A suppresses the dark current from being mixed when the incident light amount is small.
The amount of the dark current generated increases as the capacity of the potential packets increases. Therefore, if the capacity of the potential packets is decreased when the incident light amount is small and when the signal charge amount is small as described in JP 2005-286470 A, it becomes possible to decrease the amount of dark current generated and thus to increase an S/N ratio.
As described above, by decreasing the capacity of the potential packets used in the charge transfer when the incident light amount is small, it is possible to decrease the dark current. However, in JP 2005-286470 A, the capacity of the potential packets is controlled equally to increase or decrease, based on the imaging condition that the incident light amount is large or small. Therefore, for example, a dark-current component contained in a dark image portion, such as a shadow portion, of a high luminance photographic image taken under an imaging condition of a large incident light amount may become large relative to the signal charge amount. In this case, the S/N ratio can be increased in the high luminance image portion; however, the S/N ratio may decrease in a dark, low luminance image portion. As a result, it may be difficult to obtain a high quality image over the whole screen.
This phenomenon can be resolved by decreasing the capacity of the potential packets to a certain extent, under the imaging condition of the large incident light amount. However, by doing so, it is necessary to decrease a saturation signal charge amount of the high luminance image, which may narrow a dynamic range of an image.